Probing the members of globular clusters.

When a white dwarf, a type of compact star, pulls matter from a close stellar companion, periodic fireworks can result. Over the years, astronomers have found a multitude of these pairs of mercurial stars, known as cataclysmic variables. Now, scientists have for the first time obtained spectra of tightly orbiting binary stars, which may well be cataclysmic variables, within the crowded confines of a globular star cluster.

Jonathan E. Grindlay of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., Adrienne Cool of the University of California, Berkeley, and their colleagues began their study in 1992, training the unrepaired Hubble Space Telescope on the Milky Way globular cluster NGC 6397. The intensity of hydrogen emission in the Hubble images hinted that several of the stars might qualify as cataclysmic variables.

Earlier this year, the group reexamined these stars with the faint-object spectrograph aboard the refurbished Hubble. Hubble's repaired optics allow astronomers to obtain the spectra of individual stars in a crowded cluster. The spectra of three of the stars revealed emissions from hydrogen and ionized helium--a dead giveaway, according to the team, of activity associated with cataclysmic variables.

The high temperatures of these atoms and ions hint that the newly detected cataclysmic variables are magnetic, Grindlay notes. That's curious, he adds, because less than 10 percent of known cataclysmic variables have significant magnetic fields.

Strong magnetic fields are often associated with rapid stellar rotation. Grindlay speculates that in the crowded environment of a globular cluster, close encounters between neighboring stars may boost stellar rotation rates, generating large magnetic fields. Some of these stars may then evolve into magnetized white dwarfs that end up in cataclysmic variables.

The Hubble observations also suggest that cataclysmic variables are far more abundant in the cores of globular clusters than in other, lower-density regions of our galaxy, Grindlay says. If that conclusion proves true, it would have important implications for the evolution of globular clusters.

When a star in a globular cluster gets too close to a cataclysmic variable, two outcomes are possible, he notes. The cataclysmic variable may impart a gravitational kick that sends the star fleeing, or the star will trade places with and eject the white dwarf's original partner. Either way, he notes, binary stars act as cosmic eggbeaters, stirring up a globular cluster and preventing it from collapsing to form a black hole.